Well, if you only go a few times a year like you mentioned, anything should last you a long time unless you bash the hell out of it. The garage rocker is alright, I wouldn't worry about the whole freestyle description. Someday you'll evolve from just riding green runs, and on a smaller mountain you'll probably venture over to the park at some point.. and freestyle boards rock both the park and the groomers.
The quickest explanation I can give you without writing a wall of text to explain sintered vs. extruded is that extruded is slower, doesn't hold wax as well, and are usually boards you beat up pretty good. Most cheap park boards are extruded for that reason. Sintered bases are the exact opposite. Extruded = carefree. Sintered=Performance.